This invention relates to a method for treating the surface of stainless steel by high temperature oxidation.
Conventionally there has been a "metal coloring" method that allows an oxide film formed on the surface of various metals, such as aluminum, titanium or stainless steel, etc., to develop color by utilizing the phenomenon of light interference. Since this method can produce various color tones by controlling the thickness of oxide film without destroying the native brightness of the base metal, the method has been widely used on ornamental or construction materials.
The conventional methods for metal coloring comprise:
(I) Dipping metallic material in chemical reagents PA1 (II) Anodically oxidizing in chemical reagents PA1 (III) Oxidizing at elevated temperatures in an oxidizing atmosphere (refer to Japanese Laid Open Pat. Appl. Nos. 48-99047, 49-58035 & 52-134833) PA1 (a) The coating agent is composed of materials that do not melt even under the high temperature-heating of this method. As a suitable coating agent powders of TiO.sub.2 and SiO.sub.2 are mixed in a ratio between 100:0 and 25:75 in weight and the mixture is pulverized with a crusher such as a ball mill, etc. and graded by a 150-mesh sieve to achieve a small particle size, and water is added to the small-sized microparticles and to make a slip. The grade or size adjustment is performed accurately; if the slip contains some coarse particles, the oxidation film becomes uneven at locations where coarse particles contact with the metallic surface and form a speckled oxide film during the heat treatment. It has been experimentally confirmed that when the particles are adjusted to sizes smaller than 150-mesh, a slip consisting of such particles causes no unevenness in color. PA1 (b) Placing the coating agent on the exposed metal surface, after the cleaning treatment, is performed by spattering, pouring the slip or dipping the object in the slip or sprinkling the dried coating agent, etc. Among the above methods, spraying the slip is advantageous for the preadjusted slip because it gives a uniform thickness of the coating, like spraying enamel on a glass lining. One optional component of the coating material is SiO.sub.2 which can improve the spraying property of the slip. However, with increasing mixing ratio of SiO.sub.2, the adhesive strength of the dried coating decreases; accordingly, the mixing ratio of SiO.sub.2 is preferably kept under 75%. It is important to coat in such a manner that the coating has a uniformly distributed thickness after completion of the coating. PA1 (c) The pasted membrane or coating is dried completely.
Regarding (I) above, since the color tone of an oxide film varies delicately depending on the composition of the reagent and on the dipping time (the color changes with every second and every minute), the color development requires a fine control against degradation of reagents.
As to (II) above, inhomogeneities in the electric current density, or generation of oxygen gas can cause an unevenness in the coloring. Therefore, the applying material is limited to metal having simple configurations such as plates or sheets.
Colored oxide films obtained by the methods (I) (II) are subject to corrosion or abrasion because of their high porosity, and the film requires a hardening treatment after each coloring.
In method (III) above, the method is widely used for coloring materials such as stainless steels or titanium alloys having high temperature strength, because the method is easy to practice and can give a solid colored oxide film. While this method can form a colored oxide film having the tone corresponding to the heating temperature of the applying metal, it has a drawback in that it causes an unevenness or shades in color, resulting in an aggravated appearance, because the degree of oxidation differs depending on the location of matallic surface. Therefore, the use of this method has been limited to the blackening treatment of heat exchanger tubes or to small parts that require no concern for the sense of beauty.
In the food or pharmaceutical industry, the stainless steel is often used for equipment or integral parts of a factory, such as storage tanks, pipes or valves. The corrosion resistance of stainless steel is maintained, in general, by a passive film of Fe-, Cr-, Ni-oxide. However, because the thickness of the coating of only several .ANG. or tens of .ANG., the dissolution of Fe-ions cannot be avoided.
For example, in the brewing industry, sake, wine, beer, etc. contain various kinds of organic acids. In particular, in the inner or outer surface of a storage tank, ultrafiltration equipment or pipes are treated by buffing or pickling to prevent the adherence of germs or sal tartar and to improve their cleanliness. For example, the surface of ultrafiltration equipment is treated with a No. 400 mirror finish, because of the dissolution of iron into sake and the sanitary requirements of the equipment. However, when sake is stored for longer than 10 hours, iron can dissolve from the stainless steel surface into the sake, making the sake colored and lowering its commercial value from the viewpoint of its taste. Accordingly, nowadays materials for piping in such plants or the module of ultrafiltration equipment, include plastic or plastics-lined materials which are immune to the dissolution of iron.
In the pure chemical field, or the field that requires clean water such as nuclear power station or electronics industry, there are many processes that require a solution to be free from the dissolved Fe-ions.
Corrosion-resistant stainless steel is expected to increase the corrosion resistance because of a coloring process, but in practice such coloring can deteriorate the resistance, depending upon the treatment process. (Refer to Table 4 herein.) Accordingly, the coloring process can leave some problems for uses where high corrosion-resistance is required.
The reason for the deterioration of corrosion-resistance seems to be due to the fact that the oxide film formed by the heat-treatment after the mechanical abrasion is not so dense nor uniform that the base-metal cannot be subjected to crevice corrosiion or pitting corrosion.
One solution for this problem is to dip a stainless steel article having a colored oxide film formed by the high temperature oxidation, in a nitric acid solution to passivate the base metal at the defective location of the film. This process helps to prevent the corrosion resistance from becoming deteriorated to some extent, but this process has the danger of causing the dissolution of the colored oxide film resulting in a change of color tone.
It is a general object of this invention to solve the problems in the known prior art, and more specifically to provide a treatment method for forming a colored oxide film on a stainless steel surface, particularly by the use of high temperature oxidation that allows the film to have a better color and more beautiful tone with no unevenness and shades, as well as to have an improved corrosion resistance.
Furthermore, since the oxide film formed under the present high temperature oxidation treatment comprises stationary oxides of Fe, Cr, Ni having hundreds of .ANG. in thickness, the dissolution of metallic ions from the stainless steel becomes less than that from conventional oxide films. However, in the field of use where extremely strict conditions are required, the prevention from some dissolution might be unsuccessful.
Accordingly, the further object of the present invention is to prevent the dissolution from stainless steel of Fe-ions by a drastic improvement of the prior art.